Comments on: Displacement or position?

By: Li-sa

Li-sa — Wed, 26 Sep 2007 01:47:46 +0000

Displacement is a vector. Position – a point.
Displacement: change in position.

By: Ewen

Ewen — Wed, 24 Jan 2007 06:31:36 +0000

Consider 2 more arguments for naming the vertical axis “position” rather than “displacement”. The first is that if you were to use the numbers to calculate the displacement between any 2 times, you would substitute them into the formula: displacement equals position 2 subtract position 1 (not displacement 2 subtract displacement 1). The second is that when you plot the graph you are plotting positions, not displacements.
Because of these arguments, I’d also prefer to make the title of the graph “position vs. time” rather than “displacement vs. time”.

By: zac

zac — Mon, 22 Jan 2007 12:23:42 +0000

Thanks for your input, Alan.

On your last paragraph, I don’t know that I have ever heard the expressions “net displacement” or “displacement travelled”.

The displacement from the origin can be zero for a journey (we ended up where we started) or it can even be negative (since it has a direction), but the distance travelled will always be some positive value.

“Net distance travelled” or “net distance” makes more sense, surely?

By: Alan Cooper

Alan Cooper — Mon, 22 Jan 2007 08:30:54 +0000

I’m with you on the graphs. This is one of those situations where there is not a unique accepted usage and so we should not pretend there is. But I do prefer “displacement” to “position”.

In fact, to determine a coordinate, both words need to be modified by including reference to the origin (as well as unit of measurement), but to use the single word “displacement” rather than “displacement from the origin” is in my opinion less misleading. This is because even if the origin is not mentioned the use of the word “displacement” implies that it must be from something, whereas the word “position” stands alone to identify a point without begging the question “relative to what?”. (In fact the same position can be described by different displacements depending on where we put our origin.)

Where I don’t quite agree with you is on that kinematics page. The distinction between displacement and distance is just that displacement has a direction whereas distance does not, and I don’t really think that it is any less common to refer to a net or final displacement or a distance at time t than the other way around.

By: zac

zac — Mon, 22 Jan 2007 07:27:43 +0000

Well, thanks, Harro!

By: Harro!

Harro! — Mon, 22 Jan 2007 07:15:16 +0000

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